Had a dig around inside the door as already mentioned but nothing was out of place, so after yet more googling I saw a few images of how the cabling ran from the wing mirror through to the door connectors and wasn't too impressed.

So off came the wing mirror and I didn't even have to meter the cabling, I could see water had got into the cable and corroded the copper. So after weighing up the options of spending £200+ on a new wing mirror or a morning being 'creative', I have plenty of beer money to show for my efforts

Essentially I cut the corroded section out and soldered a length of cat5 in place. covered the solder joints in silicon snot and I now have electric memory folding mirrors.